Ghosts of the Past: Stromatolites

I believe in ghosts. They hide in plain sight, urging us to listen to the stories they have to tell.

It was my birthday and, feeling just a wee bit sorry for myself (I’m of that age), I decided to go in search of something older than I was—the stromatolites of the Castner Marble in the Franklin Mountains near El Paso. Now, these stromatolites aren’t just a little older than I am, they’re way older than I am. About 1.25 billion years older. Yes, that’s right: 1.25 billion.

Math isn’t the strongest of my academic skills, but let’s work our way through this. Let’s say you wanted to count to 1.25 billion—just to get a sense of how colossal that number is. If you keep up a steady rhythm of a number per second (and absolutely no stopping for dinner, a potty break, or Facebook), it will take you 40 years to reach 1.25 billion. If you wanted to restrict your counting to a mere 8 hour day, buckle down for 120 years. Hopefully, you don’t get confused, skip a number, and have to start all over. That would be depressing.

So I’m in search of 1.25 billion-year-old stromatolites. In case you’re wondering, stromatolites are a type of sedimentary rock created by single-celled photosynthetic cyanobacteria (that’s blue-green algae). These microorganisms like to congregate in large groups, forming sticky mats in shallow water along a shoreline. I don’t think anyone knows why the stickiness is necessary, but the result is that the bacterial mats accumulate sediment that clogs the system up. The bacteria, being photosynthetic, need the sun’s energy to produce food, so they move upwards, forming another mat on top of the sediment. And so it goes. Layer after layer, eventually forming a cauliflower-shaped lump of rock in shallow water.

Now the amazing thing is that these single-celled cyanobacteria have been carrying on like this for over 3.2 billion years. In fact, stromatolites are some of the oldest fossils in the world. They reached their peak diversity just about the time the Castner Marble stromatolites formed. After that, they’re harder to find. The theory is that grazers evolved that found a stromatolite mat downright tasty and pretty much ate them to near-extinction.

I cajole a geologist friend into going with me, and we head westward, driving past yucca-studded grasslands and mesquite-topped sand dunes. We skirt El Paso and climb the Trans-Mountain Highway through the Franklin Mountains. I navigate, reading directions from the Geological Excursions to a Transmountain Precambrian Adventure field guide that I’d downloaded from the internet. We’re looking for Stop 2—the Castner Marble Stromatolites. A gust of wind shakes our vehicle as we round the corner.

We pull over onto a wide gravel patch (obviously not the first geologists to investigate this location) and begin walking back down the highway to the end of the guardrail. Another gust of wind nearly knocks me off the mountain.

The authors of the field guide suggest that we follow a small trail down into the valley, and up the other side. The stromatolites will be there.

The trail is, indeed, narrow, and very steep. I cautiously make my way down, trying to avoid the baseball-sized round rocks that ensure a quick—and most likely painful—descent down the slope. I skirt the lechuguilla, prickly pear cactus, and a barrel cactus or two to arrive at a 2 to 3 ft thick band of light-colored rock.

“Are you sure these are stromatolites?” I ask Blaine. He’d gotten there before I had and was poking and prodding at the rock as geologists tend to do.

“Yeah, sure. Look.” He points to the thin, wavy layers in the rock. And there, right before me is evidence of the earliest life in the Chihuahuan Desert.

With just my naked eye I can distinguish layer after layer of sediments trapped by cyanobacteria over a billion years ago. To see the actual fossils, I’d need something slightly stronger, like a microscope. Most of the layers are flat-lying to slightly wavy—an indication that the mats collected sediments in relatively calm waters.

Stromatolites aren’t flashy like a trilobite fossil or spectacular like an ammonite. But all in all, they’re probably far more important. Stromatolites hint at the origin of life. When the cyanobacteria were trapping these sediments, the skies were pink—filled with methane and ammonia and other gasses that would kill us today. The land was barren. Not a single, living thing existed out of the water. But in the water, slimy mats of green bacteria clung together, creating their own little rock. The cyanobacteria were photosynthetic, and that nifty trick of converting the sun’s energy and some carbon dioxide into food while releasing oxygen as a byproduct was changing the world. A few billion years of lilliputian, single-celled bacteria releasing minute bubbles of oxygen created an oxygen-rich atmosphere necessary for complex life to evolve. Take a deep breath. That’s you I’m talking about.

Living stromatolites are pretty rare now, but they’re still around. In Cuatro Ciénegas, a desert wetland in the Chihuahuan Desert of northeastern Mexico, stromatolites can be found in the pozas or spring-fed pools of the marsh.

Scientists working at Cuatro Ciénegas cored one of the active stromatolites. The mat consisted of a complex community of microorganisms—not just cyanobacteria. The top layer was composed of diatoms (distinctive algae with transparent cell walls), below that was photosynthetic cyanobacteria. A third layer consisted of purple sulfur bacteria. These bacteria photosynthesize using hydrogen sulfide rather than water and produce elemental sulfur as their byproduct. Below that was a sulfur-reducing bacteria that takes the elemental sulfur and reduces it to hydrogen sulfide. “Each layer obtains and processes energy in a different way and produces different waste products, and yet it is an interactive community where each layer contributes to the survival of the mat as a whole,” the scientists concluded. Stromatolites are all about being good neighbors.

I poke around the rocks, distracted by rainbow cactus with fat buds, and a lizard warming itself in the sun. None of this would be possible without those little cyanobacteria that lived in an ancient sea long, long ago. I wave at Blaine, who’s wandered off to look at a contact or something geological and we head back to the car. Finding rocks that are billions of years older than I am, makes me happy—all in all, a pretty spectacular birthday.